Adachiite is a rare member of the tourmaline group, chemically characterized by its calcium- and iron-rich composition. It typically presents as dark, prismatic crystals found in association with metamorphic assemblages in Japan, requiring professional analysis to distinguish from more common iron-rich tourmalines like schorl.
Is this adachiite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch adachiite with a known reference. Adachiite sits at Mohs 7 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Adachiite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Adachiite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: dark green, dark brown, black.
- 5Look at form & habitCrystal system: trigonal. Typical habit: prismatic crystals with rounded triangular cross-sections.
Often confused with
Adachiite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside adachiite
Minerals reported to co-occur with adachiite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaFe₃Al₆(Si₅AlO₁₈)(BO₃)₃(OH)₃(OH)
- Mohs hardness
- 7
- Density
- 3.31 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Prismatic Crystals with Rounded Triangular Cross-sections
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Metamorphic Rocks
- Typical price
- $50-300 per specimen
Where rockhounds find adachiite
Classic worldwide localities
- Adachi mine, Fukushima Prefecture, Japan
Field-hunting tip
Look in metamorphic rocks country — that is the host setting where adachiite typically forms. If you start seeing quartz, albite, muscovite in float, you are in the right ground. Field specimens usually show a prismatic crystals with rounded triangular cross-sections habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find adachiite on the map
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